What Are Digital Power Supplies Worth?

Sam Davis | Mar 16, 2005

The future of power supplies was one of the hot topics at the Applied Power Electronics Conference, March 6-10 in Austin, Texas. One rap session and two professional education seminars tackled the subject. While nobody offered a clear picture of what this future would be like, though, some reasonable conclusions could be drawn.

For years, power supplies have been designed using analog circuit design techniques. "Digital" power supplies have been designed over the last decade as well, yet they haven't gained broad acceptance. Here, a digital supply can be defined as a supply in which a digital processor is inside the feedback loop that controls power-supply operation. That is, a processor samples the output via an analog-to-digital converter and then regulates the output voltage.

Economics are the primary reason for the digital power-supply market's lack of penetration. The major cost factor is its digital processor because the output must be sampled quickly enough to provide a stable precision output. This means you need a fast (usually relatively expensive) microprocessor or DSP with an on-chip analog-to-digital converter. Plus, the circuit requires firmware to provide the necessary control functions.

Supporting a digital power-supply design requires programmers (adding to the overall cost) as well as hardware designers. These two diverse disciplines also must be able to communicate with each other, which isn't always the case.

Assuming you can organize the proper personnel, you still have to be able to design and build the supply to meet current market needs in terms of performance and cost. Because of relatively high design costs, a digital supply would probably have to be an ac-dc type that operates at 1000 W or above. In that power range, the added cost for the digital circuits is a much smaller percentage of the overall cost—and that's not the case for supplies with lower power ratings.

An obvious question is what the digital supply offers that its analog cousin doesn't. First, it can communicate operational data efficiently with a system's host processor. If a supply's output voltage is going out of range, the digital supply can send out a fault signal. The ac-dc digital supply also could perform power factor correction as one of its time-shared duties. Second, you can change the supply's output voltage by reprogramming while using the existing hardware. And third, analog circuits can drift, but digital hardware voltages do not.